The relay may be used to cover shadow areas in a cell and installed at cell boundaries to effectively extend cell coverage and enhance throughput.
The relay may be classified into an out-band relay, in which a center frequency of a frequency band used in a backhaul link between a base station and the relay is different from a center frequency of a frequency band used in an access link between the relay and a terminal, and an in-band relay, in which the center frequencies are identical to each other.
A relay of the 3rd generation partnership project (3GPP) has been considering the time division scheme dividing the time domain for the transmission and reception to avoid self-interference (SI). The SI may occur when an identical frequency band is used for transmission and reception frequencies of the relay. That is, the SI is an interference occurring at a receiving antenna when signals are simultaneously transmitted and received at an identical frequency band at a transmitting antenna and the receiving antenna of the relay. More particularly, when a frequency band used between the relay and user equipment is identical to a frequency band used between the base station and the relay (i.e., in-band type), a signal transmitted to the user equipment through the transmitting antenna of the relay may be received by the receiving antenna itself. Thus, when the receiving antenna receives a signal from the base station, an interference may occur. Such SI may occur at not only the downlink but also the uplink.
The so-called “in-band half-duplex type” is a type of using the same frequency band and dividing the time domain for transmission and reception. An in-band half-duplex relay may receive signals from the base station (or user equipment) at a predetermined time and at a predetermined frequency at a downlink (or uplink). After performing error correction on the received signals through digital signal processing, the signals may be modulated to be a suitable transmission format and then retransmitted to the user equipment (or base station). At this time, the relay may not transmit the data to the user equipment (or base station) during the time for receiving the data from the base station (or user equipment). As such, the SI may be avoided by dividing the time domain for the transmission and reception.
In a relay of long term evolution (LTE), physical layer signals of a downlink for transmission from the base station to the user equipment may include a physical downlink shared channel (PDSCH), a physical downlink control channel (PDCCH), a physical control format indicator channel (PCFICH), a physical hybrid ARQ indicator channel (PHICH) and the like.
Since the relay may operate in the half-duplex type for avoiding the SI, it may be impossible for simultaneous transmission and reception. That is, the relay may not transmit any signal including the PDCCH through an access link during a time (period) in which the relay receives signals from the base station through a backhaul link. The relay may be able to receive data from the base station only during a time defined as a transmission gap (TG). This TG may be defined as a multimedia broadcast sing frequency network (MBSFN) in the 3GPP.
The relay may receive signals from the base station only during the time period designated as an MBSFN sub-frame defined as the TG and does not transmit any signal including the PDCCH to the user equipment during the time period. However, the relay may transmit the PDCCH to user equipment belonging to the relay by using the first and second OFDM symbols (i.e., 0th and 1st symbols) of a sub-frame designated as the MBSFN sub-frame. The relay can not receive signals of the base station during the 0th and 1st symbol periods. A normal cyclic prefix (CP) or an extended CP may be used in the 0th and 1st symbols. After transmitting the PDCCH through the 0th and 1st symbols, the relay may receive backhaul data from the base station through the same frequencies. In such a case, a switching gap for switching a transmission mode to a reception mode is required and a data starting point of a relay sub-frame is synchronized with a starting point of the backhaul data received from the base station. Further, when the reception of the backhaul data is completed, an SG for switching the reception mode to the transmission mode is required.
However, when the data starting point of the relay sub-frame is synchronized with the starting point of the backhaul data received from the base station and the length of the SG is longer than the length of the CP, the relay may have to use the last symbol period of the sub-frame as the SG for switching the reception mode to the transmission mode. As a result, a problem may arise where the relay may not receive the backhaul data of the base station during the last symbol period of the sub-frame.